Aircraft structures are built with stringers (also known as stiffeners) to increase the structure's resistance to loads, particularly bending loads. The stringers are often coupled to the wing or fuselage skin and generally extend in a span-wise direction along the wing interior or along the fuselage length. Stringers may be provided in a variety of different cross-sectional shapes and sizes including, but not limited to, an I-beam shaped cross section and/or a hat-shaped cross-section. The hat-shaped stringer is also referred to as an omega stringer. A hat-shaped or omega stringer comprises two flange portions which are bonded to the panel, from these extend two webs which extend upwardly from the flanges, the webs are connected by a cap which in combination with the webs and the panel enclose a trapezoidal cross-section.
Composite materials are widely used in aerospace applications because of their relatively light weight and favourable physical properties. One such category of composites used in aircraft manufacture is the prepreg. Prepreg is short for ‘pre-impregnated’, which refers to a fibrous reinforcement, typically a carbon fibre fabric, which is combined with a matrix material such as an epoxy resin, to form an impregnated fibre fabric which upon its use is laid up in a mould and cured to form a composite part.
Prepreg production of aircraft parts typically necessitates the lay-up to be cured in an auto-clave. The high pressure difference of the autoclave reduces the extent of porosity in the final cured part. This is necessary because pores in a composite can act as stress concentrators and are initiation sites for crack propagation. In addition, autoclaves are expensive, and also introduce size constraints into part production. Therefore, there is a preference to produce some aeroplane parts using resin transfer moulding (RTM) or a similar resin infusion process. In such processes a dry fibre preform is used in place of the prepreg, and a liquid resin is infused into a closed mould to completely wet out the dry fibre preform before curing. Infusion allows low porosity composite structures to be produced without the need for an autoclave.
Conventionally stringers are produced by first laying up prepreg that forms the panel structure onto a mould surface. An inflatable bladder or rigid mould core which defines the internal shape of the stringer is then placed on top of the panel lay-up. Further prepreg layers are placed over the bladder to form the stringer. The bladder is inflated and a caul panel placed on top of the assembly to maintain the external shape of the stringer during cure. The caul panel also distributes pressure on the surface of the assembly more evenly. The assembly is then vacuum bagged and cured in an autoclave to co-cure the hat stringer to the skin (see for example US2010007056 A1). In a similar process the stringer can be cured separately, and in a second step it is co-bonded during the cure phase of the panel.
An alternative method of forming a stringer stiffened panel is to first place a prepreg into a concave mould which defines the external shape of a stringer. A bladder which defines the internal shape of the stringer is placed on top of the prepreg, inside of the mould. Sheets of prepreg are then placed across the top of the bladder, to form the panel. Finally a flat caul panel is placed on top of the assembly. The bladder is inflated and the assembly is vacuum bagged and cured in an autoclave. The bladder applies an internal pressure to the stringer, whilst a pressure gradient over the vacuum bag consolidates the assembly. US2011084428 A1 is an example of this method.
The above methods can be adapted to be used with resin infusion processes, in which case preforms of dry-fibre reinforcement material are used in place of prepreg. The preforms are infused with a curable resin, and cured to form the stiffened stringer panel.
Stringer stiffened panels typically exhibit some fibre distortion. This is particularly pronounced when panels are made using infusion processes rather than from prepreg. This is because during infusion of the dry reinforcement, the dry reinforcement can easily be displaced, particularly where the stringer and panel reinforcement contact.
Fibre distortion occurs in particular where the stringer adjoins the panel. Here the defect manifests as parallel grooves visible on the aerodynamic surface of the panel. These defects are located where the flanges transition into the web and are no longer in contact with the panel. Because these defects are located on an aerodynamic surface it becomes necessary to fill them if they exceed a size specified by the aircraft manufacturer. Filing is usually performed by smoothing over the defect with an epoxy paste and then finishing with a surface coating. This increases the processing time, costs and weight of the part. Defects of fibre mal-alignment also result in an undesirable reduction of mechanical properties of the part.
In addition to the above mentioned problems, stringers made by infusion methods in particular can exhibit regions of over impregnation. These areas tend to be on the internal surfaces of the web and cap of the stringer or on the panel where it is contacted by the bladder.
Accordingly, it is desirable to develop a method for producing composite a stringer with fewer defects.
The present invention aims to overcome the above described problems and/or to provide improvements generally.